IP Multimedia Subsystem (IMS) Session Continuity (SC) is a technology of preventing interruption of an underway IMS session when an IMS User Equipment (UE) moves between different access networks and ensuring favorable user experience. The essence of the SC is a Service Centralization and Continuity Application Server (SCC AS), which controls session transfer of a UE between multiple networks.
An IMS Centralized Service (ICS) UE is an IMS UE with enhanced ICS capabilities. The ICS capabilities generally refer to support of a Gm interface or an I1 interface; and a non-ICS UE refers to a UE that supports neither the Gm interface nor the I1 interface, or refers to a UE that supports the Gm interface or the I1 interface but does not use such interfaces to transfer signaling in the communication process.
When a non-ICS UE transfers an ordinary voice session from a Packet-Switched (PS) domain to a Circuit-Switched (CS) domain, the non-ICS UE sends a CS domain signaling (such as setup message) in the CS domain as a transfer request, where the request carries a Session Transfer Number (STN) as a called number; after receiving the request, the SCC AS determines the request as a transfer request according to the STN value, and correlates the related PS domain session, updates the peer, and releases the relevant session in the PS domain to finish the transfer process.
The multi-session transfer of the non-ICS UE from the PS domain to the CS domain supports only transfer of two sessions. Before sending a transfer request, the UE releases other sessions than the two sessions to be transferred. If the UE fails in releasing other sessions, the SCC AS needs to correlate the transfer request with the sessions to be transferred after receiving the transfer request. If more than two sessions exist at this time, the SCC AS can also initiate release of the sessions. The detailed solution is: The active session is transferred first (if no session is active, only the lately held session needs to be transferred); after completion of the transfer, the SCC AS sends information about an active session which is activated before the lately activated session, or information about the lately held session, to a Mobile Switching Center (MSC) server, and the MSC server initiates transfer of a second session in place of the UE.
In the process of researching and practicing the prior art, the inventor of the present invention finds that: When the UE moves from a PS network to a CS network and the UE involves multiple sessions, if the second session transferred is a multimedia session, the prior art does not take into account of abnormality that the second session may be not transferable because the current Radio Access Network (RAN) of the UE does not support video media transmission or the current network resources are not enough for supporting the video media transmission.